Coal is the world's most abundant fossil fuel. However, coal has three major drawbacks: (1) Coal is a solid and is less easily handled and transported than fluidic or gaseous materials; (2) Coal contains compounds which, on burning, produce the pollutants associated with acid rain; and (3) Coal is not a uniform fuel product, varying in characteristics from region to region and from mine to mine.
In fossil fuels, the ratio of hydrogen atoms to carbon atoms is most important in determining the heating value per unit weight. The higher the hydrogen content, the more liquid (or gaseous) the fuel, and the greater its heat value. Natural gas, or methane, has a hydrogen-to-carbon ratio of 4 to 1 (this is the maximum); coal has a ratio of about 1 to 1; shale oil about 1.5 to 1; petroleum crude about 2.0 to 1; and gasoline almost 2.2 to 1.
The lignites, peats, and lower calorific value subbituminous coals have not had an economic use except in the vicinity of the mine site, for example, mine mouth power generation facilities. This is due primarily to the cost of shipping a lower Btu product as well as to the danger of spontaneous combustion because of the high content of volatile matter and high percentage of moisture which is characteristic of such coals.
Since low-rank coals contain high percentages of volatile matter, the risk of spontaneous combustion is increased by dehydration, even by the non-evaporation methods. Therefore, in order to secure stability of the dehydrated coal in storage and transportation, it has been necessary to cover the coal with an atmosphere of inert gas such as nitrogen or combustion product gas, or to coat it with crude oil so as not to reduce its efficiency as a fuel. However, these methods are not economical.
Waste coal has somewhat different inherent problems from those of the low-rank coals. Waste coal is sometimes referred to as a "non-compliance coal" because it is too high in sulfur per unit heat value to burn in compliance with the United States Environmental Protection Agency (EPA) standards. Other waste coal is too low in Btu to be transported economically. This coal represents not only an environmental problem (because it must be buried or otherwise disposed of), but also is economically unattractive.
The inefficient and expensive handling, transportation and storage of coal (primarily because it is a solid material) makes coal not economically exportable and the conversion of oil-fired systems to coal less economically attractive. Liquids are much more easily handled, transported, stored and fired into boilers.
Coal transportation problems are compounded by the fact that coal is not a heterogeneous fuel, i.e, coal from different reserves has a wide range of characteristics. It is not, therefore, a uniform fuel of consistent quality. Coal from one region (or even of a particular mine) cannot be efficiently combusted in boilers designed for coal from another source. Boilers and pollution control equipment must either be tailored to a specific coal or configured to burn a wide variety of material with a loss in efficiency.
The non-uniformity and transportation problems are compounded by combustion pollutants inherent in coal. Coal has inherent material which, upon combustion, creates pollutants which are thought to cause acid rain; specifically, sulfur compounds and nitrogen compounds. The sulfur compounds are of two types, organic and inorganic (pyritic). The fuel bound nitrogen, i.e., organic nitrogen in the coal, combusts to form NO.sub.x. Further, because of the non-uniformity of coal it combusts with "hot spots". Some of the nitrogen in the combustive air (air is 75% nitrogen by weight) is oxidized to produce NO.sub.x as a result of the temperature created by these "hot spots". This so-called "thermal NO.sub.x " has heretofore only been reduced by expensive, coal-fired, boiler modification systems.
Raw coal cleaning has heretofore been available to remove inorganic ash and sulfur but is unable to remove the organic nitrogen and organic sulfur compounds which, upon combustion, produce the SO.sub.x and NO.sub.x pollutants. Heretofore fluidized bed boilers, which require limestone as an SO.sub.x reactant, and scrubbers or NO.sub.x selective catalytic converters (so-called combustion, and post-combustion clean air technologies) have been the main technologies proposed to alleviate these pollution problems. These devices clean the combustion and flue gas rather than the fuel and are tremendously expensive from both capital and operating standpoints, adding to the cost of power. This added power cost not only increases the cost of domestically produced goods, but also ultimately diminishing this nation's competitiveness with foreign goods. Further, this inefficiency also produces more CO.sub.2. CO.sub.2 production has been linked by some with "global warming", i.e. an increase in the "greenhouse" effect.
It would, therefore, be advantageous to clean up the coal by removing the organic nitrogen (fuel nitrogen), as well as the organic sulfur while providing a uniform fuel with high reactivity and lower flame temperature to reduce the thermal NOhd x. In order to overcome some of the inherent problems with coal, various methods have been proposed for converting coal to synthetic liquid or gaseous fuels. These "synfuel" processes are capital intensive and require a great deal of externally supplied water and external hydrogen, i.e., hydrogen and water provided from other than the coal feedstock. The processes are also energy intensive in that most carbon atoms in the coal matrix are converted to hydrocarbons, i.e., no char. The liquefaction of coal involves hydrogenation using external hydrogen. This differs markedly from merely "rearranging" existing hydrogen in the coal molecule as in hydrodisproportionation.
Coal pyrolysis is a well-known process whereby coal is thermally volatilized by heating the coal out of contact with air. Different pyrolysis products may be produced by varying the conditions of temperature, pressure, atmosphere, and/or material feed. Thus, traditional pyrolysis is the slower heating of coal in the absence of oxygen to produce very heavy hydrocarbon tars and carbon (char) with the liberation of hydrogen.
In prior art pyrolysis, the coal is heated relatively slowly at lower heating rates and longer residence times such that the solid organic material undergoes a slow decomposition of the coal molecule at reaction rate k.sub.1 to yield "decomposition" products, primarily free radical hydrocarbon pieces or fragments. These "decomposition" products undergo a rapid recomposition or "condensation" reaction at reaction rate k.sub.2. The condensation reaction produces char and dehydrogenated hydrocarbons, thus liberating hydrogen and heavy (tarry) liquids. The decomposition reaction is not desirable in a refining type process because it liberates hydrogen (instead of conserving it) and produces heavy material and char. In prior art pyrolysis, when heating is slower such that k.sub.1 (relatively slow reaction rate) and k.sub.2 (relatively more rapid reaction rate) overlap, the dehydrogenation of the decomposition product, i.e., condensation reaction, is predominant. Because it is believed that unless the decomposition reaction take place rapidly (k.sub.1 is large), this reaction and the condensation reaction will take place within the particle where there is little hydrogen present to effect the hydrogenation reaction.
Hydropyrolysis of coal to produce char, liquids, and gases from bituminous and subbituminous coals of various ranks attempted to add hydrogen such that decomposition products were hydrogenated. This process is sometimes called "partial liquefaction" and has been carried out in both the liquid and gaseous phases. As used herein, "partial liquefaction" is meant to include all thermally based coal conversion processes, whether catalyzed or not, wherein a partial pressure of hydrogen is present. The most economical of these processes take place under milder conditions. These processes have had only limited success. Without rapid heating rates, the decomposition material can not be hydrogenated by external hydrogen without use of extreme temperatures and pressures. These processes are known as "liquefaction".
In these so-called "liquefaction" processes, coal is treated with hydrogen to produce petroleum substitutes. These processes have been known for many years. Typically, these processes have mixed crushed coal with various solvents, with or without catalysts; heated the mixture to reaction temperature; and reacted the coal and hydrogen at high pressure and long residence times. These "liquefaction" processes require high pressure, usually above 2,000 psig; require long reaction residence times, 20 minutes to about 60 minutes; consume large quantities of expensive externally generated hydrogen; and produce large amounts of light hydrocarbon gases. Solvent addition and removal, catalyst addition and removal, high pressure feed system, high pressure long residence time reactors, high hydrogen consumption, and high pressure product separation and processing have made these processes uneconomical in today's energy market.
Partial liquefaction of coal by hydropyrolysis to produce char and pyrolysis liquids and gases from bituminous and subbituminous coals of various ranks attempted to add hydrogen such that decomposition products were hydrocracked. These processes have had only limited success.
In order to promote hydrogenation, more stringent reaction conditions were required, reducing the economic viability. Examples of such processes are disclosed in U.S. Pat. Nos. 4,704,134; 4,702,747; and 4,475,924. In such processes, coal is heated in the presence of hydrogen or a hydrogen donating material to produce a carbonaceous component called char and various hydrocarbon-containing oil and gas components. Many hydropyrolysis processes employ externally generated additional hydrogen which substantially increases the processing cost and effectively makes the process a "liquefaction" process.
A particular type of coal hydropyrolysis, flash hydropyrolysis, is characterized by a very short reactor residence time of the coal. Short residence time (SRT) processes are advantageous in that the capital costs are reduced because the feedstock throughput is so high. In SRT processes, high quality heat sources are required to effect the transformation of coal to char, liquids and gases.
In many processes, hydrogen is oxidized within the reactor to gain the high quality heat. However, the oxidation of hydrogen in the reactor not only creates water but also reduces the hydrogen available to hydrogenate hydrocarbons to higher quality fuels. Thus, in prior art processes, either external hydrogen is required or the product is degraded because valuable hydrogen is converted to water.
The prior art methods of deriving hydrogen for hydropyrolysis or partial liquefaction are either by: (1) purchasing or generating external hydrogen, which is very expensive; (2) steam-methane reforming followed by shift conversion and CO.sub.2 removal as disclosed in a paper by J. J. Potter of Union Carbide; or (3) char gasification with oxygen and steam followed by shift conversion and CO.sub.2 removal as disclosed in a paper by William J. Peterson of Cities Service Research and Development Company.
All three of these hydrogen production methods are expensive, and a high temperature heat source such as direct O.sub.2 injection into the hydropyrolysis reactor is still required to heat and devolatilize the coal. In the prior art processes, either carbon (char) is gasified by partial oxidation such as in a Texaco gasifier (U.S. Pat. No. 4,491,456 to Schlinger and U.S. Pat. No. 4,490,156 to Marion et al.), or oxygen was injected directly into the reactor. One such system is disclosed in U.S. Pat. No. 4,415,431 (1983) of Matyas et al. When oxygen is injected directly into the reactor, it preferentially combines with hydrogen to form heat and water. Although this reactor gives high-quality heat, it uses up hydrogen which is then unavailable to upgrade the hydrocarbons. This also produces water that has to be removed from the reactor product stream and/or floods the reactor. Additionally, the slate of hydrocarbon co-products is limited.
Thus, it would be advantageous to have a means for producing: (1) a high-quality heat for volatilization, (2) hydrogen, and (3) other reducing gases prior to the reaction zone without producing large quantities of water and without using up valuable hydrogen.
Flash hydropyrolysis, however, also proved to have substantial drawbacks in that the higher heating rates needed for short residence time tend to thermally hydrocrack and gasify the material at lower pressures. This gasification reduces liquid yield and available hydrogen. Thus, attempts to increase temperature to effect flash reactions tended to increase the hydrocracking of the valuable liquids to gases.
In U.S. Pat. Nos. 4,671,800; 4,658,936; 4,832,831; and 4,878,915, it is disclosed that coal can be subjected to pyrolysis or hydropyrolysis under certain conditions to produce a particulate char, gas and a liquid organic fraction. The liquid organic fraction is rich in hydrocarbons, is combustible, can be beneficiated and can serve as a liquid phase for a carbonaceous slurry fuel system. The co-product distribution, for example, salable hydrocarbon fractions such as BTX and naphtha, and the viscosity, pumpability and stability of the slurry when the char is admixed with the liquid organic fraction are a function of process and reaction parameters. The rheology of the slurry is a function of solids loading, sizing, surfactants, additives, and oil viscosity.
Common volatilization reactors include the fluidized bed reactor which uses a vertical upward flow of reactant gases at a sufficient velocity to overcome the gravitational forces on the carbonaceous particles, thereby causing movement of the particles in a gaseous suspension. The fluidized bed reactor is characterized by large volumes of particles accompanied by long, high-temperature exposure times to obtain conversion into liquid and gaseous hydrocarbons. Thus, this type of reactor is not very conducive to short residence time (SRT) processing and may produce a large quantity of polymerized (tarry) hydrocarbon co-products.
Another common reactor is the entrained flow reactor which utilizes a high-velocity stream of reactant gases to impinge upon and carry the carbonaceous particles through the reactor vessel. Entrained flow reactors are characterized by smaller volumes of particles and shorter exposure times to the high-temperature gases. Thus, these reactors are useful for SRT-type systems.
In one prior art two-stage entrained flow reactor, a first stage is used to react carbonaceous char with a gaseous stream of oxygen and steam to produce hydrogen, oxides of carbon, and water. These products continue into the second stage where volatile-containing carbonaceous material is fed into the stream. The carbonaceous feed reacts with the first-stage gas stream to produce liquid and gaseous hydrocarbons, including large amounts of methane gas and char.
Prior art two-stage processes for the gasification of coal to produce primarily gaseous hydrocarbons include U.S. Pat. Nos. 4,278,445 to Stickler; 4,278,446 to Von Rosenberg, Jr.; and 3,844,733 to Donath. U.S. Pat. No. 4,415,431 issued to Matyas et al. shows use of char as a carbonaceous material to be mixed with oxygen and steam in a first-stage gasification zone to produce a synthesis gas. Synthesis gas, along with additional carbonaceous material, is then reacted in a second-stage hydropyrolysis zone wherein the additional carbonaceous material is coal to be hydropyrolyzed.
U.S. Pat. No. 3,960,700 to Rosen describes a process for exposing coal to high heat for short periods of time to maximize the production of desirable hydrocarbons.
One method of terminating the volatilization reaction is by quenching the products either directly with a liquid or gas, or by use of a mechanical heat exchanger. In some cases, product gases or product oil are used. Many reactors, including those for gasification have employed a quench to terminate the volatilization reaction and prevent polymerizing of unsaturated hydrocarbons and/or gasification of hydrocarbon products. Some have employed intricate heat-exchange quenches, for example, mechanical devices to attempt to capture the heat of reaction. One such quench scheme is shown in U.S. Pat. No. 4,597,776 issued to Ullman et al. The problem with these mechanical quench schemes is that they introduce mechanical heat-exchanger apparatus into the reaction zone. This can cause tar and char accumulation on the heat-exchanger devices, thereby fouling the heat exchanger.
Thus, if the coal has a hydrogen-to-carbon ratio of 1, and if the hydrogens on half the carbons could be transferred or "rearranged" to the other half of the carbons, then the result would be half the carbons with 0 hydrogens and half with 2 hydrogens. The first portion of carbons (with 0 hydrogens) is char; the second portion of carbons (with 2 hydrogens) is a liquid product similar to a petroleum fuel oil. If this could be accomplished using only hydrogen inherent in the coal, i.e., no external hydrogen source, then the coal could be refined in the same economical manner as petroleum, yielding a slate of refined hydrocarbon products and char.
It would be highly advantageous to have a fuel system which is easily and efficiently prepared solely from coal using no external water and producing a slate of clean burning, non-"acid rain" producing co-products, petroleum substitutes, and chemical feedstocks including benzene, toluene, xylene (BTX); ammonia; sulfur; naphtha; gasoline; diesel fuel; jet fuel; and the like.
Further, it would be highly advantageous to have a partial liquefaction process for refining coal wherein short residence times and internally generated hydrogen are used in mild conditions to efficiently produce larger quantities of hydrocarbon liquids without excess gasification of such products by high temperatures. In this manner, hydrogen in the coal could be preserved and maximized.